Trunked communication systems, such as cellular communication systems, are well known to include a fixed system controller, a plurality of fixed base sites, and a plurality of mobile subscriber units, such as radios, radiotelephones, or portable wireless data terminals. Each fixed base site is coupled to the fixed system controller through a wireline or microwave link and provides communication service to a respective service coverage area. The fixed system controller assigns a predetermined number of communication channels, or resources, to each fixed base site. When a mobile subscriber unit desires to communicate, the subscriber unit transmits a channel request via a control channel to the fixed base site serving the coverage area containing the subscriber unit. The base site forwards the request to the system controller and the system controller grants the channel request if at least one of the channels assigned to the base site is available. Upon granting, or allocation, of the channel, the system controller informs the subscriber unit of the allocation via the base site and establishes any necessary switching connections to connect the base site to another base site or a switching center that is coupled to the public switched telephone network (PSTN). Upon receiving the channel allocation, the subscriber unit begins transmitting and receiving its communication via the allocated channel.
If, during the communication, the subscriber unit travels, or is about to travel, into another service coverage area, the system controller detects this condition in accordance with known techniques and initiates a procedure to transfer, or handoff, the communication from the current channel to a new channel assigned to the base site (target base site) serving the coverage area that the subscriber unit has ventured, or is about to venture, into. The system controller selects the new channel and informs the subscriber unit of the new channel allocation via a transmission over the control channels of the original and target base sites.
In many metropolitan areas, common carrier transportation devices, such as trains, buses, and cars, are used to provide mass transportation to the populations of these areas. In such devices, many commuters may be using their mobile subscriber units simultaneously to participate in communications. Thus, when the common carrier device moves from one service coverage area to another in the cellular system, many handoffs must occur, in effect, simultaneously in order to allow the ongoing communications to continue. As noted above, each new channel allocation associated with a handoff is conveyed individually to each subscriber unit. Accordingly, in the mass commuter handoff situation, many control channels are needed simultaneously to perform the required number of handoffs. However, each fixed base site is assigned a fixed number of control channels and, at the time of mass handoff, may have less than the fixed number of control channels available for use by the mobile subscriber units. Therefore, when the number of subscriber units to be handed off exceeds the number of available control channels, the fixed system controller drops, or disconnects, the communications of the excess subscriber units. Consequently, as described, mass handoff in existing cellular systems is a low reliability operation.
Therefore, a need exists for a method and apparatus for providing communication service to communication units located in a common carrier transportation device that provides highly reliable communication service to a large number of mobile subscriber units that are simultaneously communicating while traveling in the common carrier transportation device. Further, such a method and apparatus that substantially reduces control channel activity during a mass handoff would be an improvement over the prior art.